1. Field of the Invention
This invention relates to a process for the controlled polymerization of stereospecific alpha-olefins having a preselected isotacticity.
2. Description of Art
Polypropylene manufacturing processes typically involve the polymerization of propylene monomer with an organometallic catalyst of the Ziegler-Natta type. The Ziegler-Natta type catalysts polymerize the propylene monomer by an anionic coordination mechanism to produce solid, crystalline polypropylene. Many desirable product properties, such as strength and durability, depend on the crystallinity of the polypropylene, which in turn is dependent on the stereospecific arrangement of methyl groups on the polymer backbone. A form of the polymer in which the methyl groups are aligned on the same side of the polymer chain is known as isotactic polypropylene, as opposed to atactic polypropylene in which the methyl groups are randomly positioned.
In earlier processes, the polymerization has been conducted in the presence of inert diluents, such as heptane or xylene. Catalyst and liquified propylene are fed into the diluent and the resulting reaction produces polymer granules which form a slurry with the diluent. After reaction, the polymer has to be freed from excess diluent and propylene monomer and washed to remove residual catalyst and atactic material. Subsequent processes have eliminated the diluent by using both liquefied propylene as a slurry medium and more active, stereospecific catalysts. Development of more highly active catalyst systems has further reduced the necessity for, and in many cases allowed elimination of, the washing-drying steps.
Recent processes have eliminated diluents and slurry media by conducting propylene polymerization in the gas phase in stirred or fluidized bed reactors. Highly active, stereospecific catalysts are now commonly used and catalysts with productivities of over 30 kg of resin per gram of catalyst and selectivities of greater than 97% isotactic polypropylene have been developed. These catalysts can substantially eliminate the need for catalyst residue and atactic polypropylene removal steps.
Catalytic components that have been employed in the industrial manufacture of alpha-olefin polymers such as propylene, butene-1, etc., include a solid component comprising at least magnesium, titanium and chlorine and an activating organoaluminum compound. These may be referred to as supported coordination catalysts or catalyst systems. The activity and stereospecific performance of such compositions is generally improved by incorporating an electron donor (Lewis base) in the solid component and by employing as a third catalyst component a selectivity control agent.
For convenience of reference, the solid titaniumcontaining constituent of such catalysts is referred to herein as the "catalyst". The organoaluminum compound, whether used separately or partially or totally complexed with a selectivity control agent, is referred to herein as the "cocatalyst" or "alkyl." The selectivity control agent compound, whether used separately or partially or totally complexed with the organoaluminum compound, is referred to herein as the "SCA".
Supported coordination catalysts of this type are disclosed in numerous patents. See, for example, U.S. Pat. Nos. 4,226,741; 4,329,253 and published European patent application No. 19,330. Catalyst systems of this type which have been disclosed in the prior art generally are able to produce olefin polymers in high yield and, in the case of catalysts for polymerization of propylene or higher alpha-olefins, with high selectivity to stereoregular polymer.
The objective of workers in this art is to provide catalyst systems which exhibit sufficiently high activity to permit the production of polyolefins in such high yield as to obviate the necessity of extracting residual catalyst components. In the case of propylene and higher olefins, an equally important objective is to provide catalyst systems of sufficiently high selectivity toward isotactic stereoregular products to obviate the necessity of extracting atactic polymer components. Further, it is important that the resulting poly(alpha olefin) have other acceptable properties such as a melt flow index between between 0.1 and 1000. "Melt flow index" may be defined as the number of grams of polymer resin at 230.degree. C. that can be forced through a 2.0955 mm orifice in 10 minutes by a 2160 gram force.
Although many chemical combinations provide active catalyst systems, practical considerations have led workers in the art to concentrate on certain preferred components. The solid component of the catalyst typically comprises magnesium chloride, titanium chloride (generally in tetravalent form) and, as an electron donor, an aromatic ester such as ethyl benzoate or ethyl p-toluate. The cocatalyst typically is an aluminum trialkyl such as aluminum triethyl or aluminum tri-isobutyl, often used at least partially complexed with a selectivity control agent or agents. The selectivity control agent typically is an aromatic ester such as ethyl-paramethoxybenzoate (ethyl anisate).
Catalysts for the manufacture of stereospecific alphaolefin polymers include those described in U.S. Pat. Nos. 4,442,225 to Takitani et al.; 4,563,512 to Goodall; 4,414,132 to Goodall et al.; 4,483,966 to Suzuki et al.; and 3,112,300 and 3,112,301 to Natta et al.
In a continuous reaction system, the reaction mixture is typically maintained at conditions at which the polymer is produced as a slurry of powder in the reaction mixture. Use of highly active and highly stereospecific catalyst systems in propylene polymerization substantially eliminates the need to remove catalyst components or atactic polymer from the polymer product. The mixture of other components fed continuously or at frequent intervals into the reactor system must be monitored so as to ensure an efficient reaction and the desired product. For example, it is well known that supported coordination catalysts and catalyst systems of the type described above are highly sensitive, in varying degrees, to catalyst poisons such as water, oxygen, carbon oxides, acetylenic compounds and sulfur compounds.
The total amount of aluminum alkyl compounds in the polymerization reaction mixture is generally in the range from 10 to 200, and in most cases between 30 and 130, moles per atom of titanium in the catalyst. Differently prepared catalysts vary in the Al:Ti ratio required for best results as will be known to persons familiar with this type of catalyst. In general, activity may be greater at higher Al:Ti ratios, but this results in undesirable higher aluminum residues in the polymer; it also tends to increase the requirement of selectivitY control agent in order to maintain the desired degree of isotacticity of the product. The desired balance of concentration of catalyst components generally has been determined empirically.
It is generally possible to control catalyst productivity and product isotacticity within limits, by adjusting the molar feed ratio of alkyl to selectivity control agent (SCA). Increasing the amount of SCA increases selectivity to isotactic or stereoregular polymer, but may reduce activity, and hence catalyst productivity. Attempts have been made to monitor the selectivity of the process to the manufacture of isotactic polypropylene by directly measuring the Isotactic Index (II) or the Xylene Solubles (XS) of the polypropylene product.
Selectivity to isotactic polypropylene is typically determined under the XS test by measuring the amount of polypropylene materials which are xylene soluble, in accordance with regulations of the U.S. Food and Drug Administration. The XS test is carried out as follows: A sample product of the propylene polymerization process is completely dissolved in xylene, which contains oxidation inhibitor, in a stirred flask by heating under reflux at 140.degree. C. The flask then is immersed in a water bath at 25.degree. C. without stirring for one hour, during which the insoluble portion precipitates. The precipitate is filtered off and the solubles present in the filtrate are determined by evaporating a 100 ml aliquot of the filtrate, drying the residue under vacuum, and weighing the residue. The xylene-solubles consist of amorphous material with some low molecular weight crystalline material (FDA regulations 121.2501 and 121.2510, 1971).
The Isotactic Index (II), on the other hand, measures the amount of polypropylene material insoluble in n-heptane. Although the two tests, XS and II, are generally run using different solvents, they generate results which are predictably related since one test (XS) measures insolubility while the other (II) measures solubility. The Xylene Solubles of polypropylene is related to the Isotactic Index by the relationship XS%=63.2-0.629 (II%).
Although both XS and II can thus be measured- directly using known laboratory sampling techniques, and the reaction adjusted accordingly to obtain optimum isotacticity, such tests ordinarily require a relatively long time to run, on the order of six to eight hours for XS and on the order of six to 24 hours for II. In systems of the prior art, the catalytic process can thus be producing polypropylene not having the desired isotacticity during the long testing periods. Furthermore, in these prior art systems, awaiting the results of adjustments to the reaction will require additional time as the II or XS tests must be run again once corrections have been made. Thus, many hours can go by awaiting XS and II test results during which time large quantities of unacceptable or non-optimum resins may be produced.